Hydrogels are three dimensional, hydrophilic, polymeric networks which swell in water without dissolving and retain large quantities of water. They typically comprise both natural polymers such as starches and cellulose derivatives and synthetic polymers and copolymers such as polyethylene glycol and poly(glutamic acid). The networks within the hydrogel can be crosslinked either chemically or physically by cohesive bonds such as ionic interaction, hydrogen bonding or hydrophobic interactions. Alginates and chitosans are examples of polymers that are cross-linked by ionic interactions. Hydrogels are suitable for numerous applications such as implants, contact lenses, membranes for isosensors and drug delivery devices. Drugs may be incorporated in the hydrogel matrix by either mixing the copolymer together with the drug prior to the addition of the crosslinker which initiates polymerization or by placing the preformed hydrogel in the drug solution until it swells to equilibrium.
Hydrogels can be used to release drugs slowly over time, to protect drugs from degradation or to trigger drug release in response to various stimuli such as, for example, temperature, insulin blood levels or inflammation. Due to their high water content and soft and elastic consistency, hydrogels resemble natural tissue causing minimal mechanical irritation.
However, hydrogels typically are found to be unsuitable as controlled drug delivery devices for low molecular weight hydrophilic compounds because the high water content of the hydrogel and the presence of large pores results in rapid drug release.
There exits a need in the art for hydrogel compositions which are useful for the controlled release administration of low molecular weight therapeutically active agents and other therapeutically active agents which have been found to be difficult to administer over a prolonged period of time with hydrogels.